"But down the line what's often likely to occur is that they might get pelvic inflammatory disease, that's effectively pain in the pelvic region. Following that, what it can lead to is infertility.

"So, many young women are likely to want to get pregnant in the future, and they might be precluded from doing that because they had chlamydia in the past."

Professor Wilson's research colleague Carol el-Hayek has analysed data from five states over three years.

She found 13 per cent of 12 to 15-year-old girls tested for sexually transmitted infections carried chlamydia.

"Twelve to 15-year-olds are sexually active. The fact that they're testing for STIs or that doctors are testing them for STIs means that they are practising sex, and they're probably practising unsafe sex," she said.

Safe sex message falls off the radar

Professor Wilson says there has been a substantial trend towards people having sex at a younger age.

"I think our main messaging through schools, but more importantly through the home, through parents, and then through friends, the social media and other educational messages are not getting through most appropriately," he said.

Dr Anna McNulty from the Sydney Sexual Health Centre says chlamydia is easy to detect, easy to test for and easy to treat.

"I would encourage all young people who are sexually active to either see their GP or find a service; Google sexual health and find their closest service and find their closest service and seek some testing," she said.

One of Sydney's biggest fertility clinics, Sydney IVF, says the safe sex message has fallen from the public radar.

The clinic says it is expecting fallopian tube-related infertility to steadily increase over the next few years as a result of increased chlamydia infections.

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